In general, gas insulated switchgears are widely used to provide switching function in electric power systems for power transmission, distribution, and receiving or servicing. Main circuit conductors in gas insulated switchgears are arranged in a phase-separation configuration or a three-phase-in-one-housing layout.
For measuring the current flowing through each of the main circuit conductors in the phase-separation configuration, a current transformer container is provided adjacently to a bus container in which the main circuit conductor is accommodated in taut condition. The current transformer container is comprised of an inner cylinder and an outer sealing member and the current transformer is installed on the outer circumference of the inner cylinder. The current transformer is installed in multiple as the capacity of the gas insulated switchgear increases.
The current transformer is a window-type instrument current transformer, in which the main circuit conductor is the single-turn primary conductor thereof and the secondary winding of the current transformer installed on the outer circumference of the inner cylinder is the secondary conductor thereof. To the secondary winding of the current transformer, a current measuring circuit is connected as the secondary circuit for metering the current flowing through the main circuit conductor detected by the current transformer. The current transformer container of this fashion has been described in, for example, Japanese patent application JP 2003-164024 A1 (Literature 1).
In the meantime, the bus container, which houses the main circuit conductor, and the current transformer container are grounded from the viewpoint of the safety assurance. This configuration makes the inner cylinder and the outer sealing member of the current transformer container form a closed loop. As the consequence of this, current induced by the main circuit conductor flows in the current transformer container (the inner and the outer sealing members) with the accuracy of the current measuring with the current transformer reduced. To prevent this, an insulation gap is formed between one longitudinal end of the inner cylinder and the bus container.
In a gas insulated switchgear by the way, a large switching surge that appears on the switching operation of a circuit breaker, which is a disconnecting switch, or of a earthing switch propagates through the main circuit conductor in a form of a traveling wave. This traveling wave induces a high-frequency surge voltage in the bus container that confronts the main circuit conductor according to the inductance of the bus container, static capacity between the main circuit conductor and the bus container, and other particulars related thereto.
Propagation of induced surge voltage traveling in the bus container is disturbed by an insulation gap formed between the inner cylinder of the current transformer container and the bus container. This disturbance of propagation generates a surge voltage between the outer sealing member of the current transformer container and the main circuit conductor. Thus, the current transformer becomes to be exposed to the surge voltage appeared between the outer sealing member of the current transformer container and the main circuit conductor and is coupled to the surge voltage through stray capacities. Then, a surge voltage is induced in the current transformer and the induced surge voltage propagates into the secondary circuit.
The magnitude of the surge voltage induced in the current transformer reaches tens of thousands of volts at its peak with frequency components of several MHz to several tens of MHz depending on the rated voltage of the gas insulated switchgear, the diameter of the current transformer container, and other elements.
Conventionally, a surge absorber such as a varistor is provided within the current metering circuit, which is the secondary circuit, to protect electronic parts composing the current metering circuit from the surge voltage.